The use of composite materials as replacements for metals in the manufacture of various components continues to increase, at least in part, due to composite materials' high strength-to weight ratio as compared to metals. Polymer matrices in composite materials include various reinforcing materials including: glass, steel, aramid, boron, carbon, etc., often in the form of fibers. The fibers afford the polymers increased strength and stiffness along the axis of reinforcement.
Thermoplastic polymer resin systems provide desirable characteristics in the manufacture of various composite materials. However, studies have determined that as melted thermoplastic materials cool, grain boundaries forming in the solidifying crystalline thermoplastic structure can impact mechanical properties in resulting composite materials comprising such thermoplastic resin systems. During thermoplastic resin cooling, grain boundaries form as a result of random crystal orientation that occurs upon cooling. Complex processing protocols relative to temperature control have been attempted to minimize grain boundaries. However, such complex temperature control protocols further complicate tooling and cooling system design, and further limit the variety and type of parts that can be produced using, for example, crystalline thermoplastic carbon fiber reinforced polymers (CFRPs).
For example, one protocol used to minimize grain boundary formation involves cooling a melted polymer from above its melting temperature to 30° F. below its melting temperature over the period of five minutes, then hold the polymer in a 10° F. temperature band around this lower temperature for 120 minutes. While such a protocol can be accomplished and controlled on a laboratory scale test sample, it presents significant challenges in part production settings where the part is large. Accurately and rapidly cooling a large part of varying geometry and then halting that cooling is a significant challenge.